create table tst as
select rownum id,
         round(dbms_random.value(1, 2)) || '.' || round(dbms_random.value(1, 2)) cod,
         round(dbms_random.value(1, 100)) sm
   from dual
connect by level < 1000001;
-- для появления разницы в скорости между вариантами запросов можно добавить до 10-ка млн записей

 with v_dict as (select '1' cod from dual union all
                 select '1.1' cod from dual union all
                 select '1.2' cod from dual union all
                 select '2' cod from dual union all
                 select '2.1' cod from dual union all
                 select '2.2' cod from dual)
select d.cod, sum(sm)
  from tst t,
       v_dict d
 where t.cod like d.cod || '%'
 group by d.cod
 order by d.cod;

with t as (select regexp_substr(cod, '^\d') cod_level1, cod, sm
             from tst)
select coalesce(t.cod, cod_level1) cod, sum(sm)
  from t
 group by grouping sets(cod_level1, (cod_level1, cod))
 order by coalesce(t.cod, cod_level1);

with t as (select substr(cod, 1, 1) cod_level1, cod, sm
             from tst)
select coalesce(t.cod, cod_level1) cod, sum(sm)
  from t
 group by grouping sets(cod_level1, (cod_level1, cod))
 order by coalesce(t.cod, cod_level1);